Electron tube filament support structure employing deformable loop portions of the filaments



Aug. 4', 1970 J. A. BAKER 3,522,457

ELECTRON TUBE FILAMENT SUPPORT STRUCTURE EMPLOYING DEFORMABLE LOOPPORTIONS OF THE FILAMENTS Filed Oct. 26, 196' 2 Sheets-Sheet l 42 I soINVENTOR.

JEAN A. BAKER BY M1 249? ATTORNEYS Aug. 4, 1970 J. A. BAKER 3,522,457

ELECTRON TUBE FILAMENT SUPPORT STRUCTURE EMPLOYING DEFORMABLE LOOPPORTIONS OF THE FILAMENTS Filed Oct. 2'3, 1967 2 Sheets-Sheet 2 r4 7 a70 (N (6) 2 v 64 64 INVENTOR. 1 f2 JEAN A. BAKER I BYW ATTORNEYS UnitedStates Patent U.S. Cl. 313278 16 Claims ABSTRACT OF THE DISCLOSURE Anelectron tube filament structure having a filament wire and a filamentsupport. The filament wire has a bight looped over one member of thesupport and two rectilinear strands extending between the bight andanother member of the support. The supporting members are spaced andbiased from each other to hold the strands taut. The bight dampensdeviations in strand tension through bowing action confined to a planesubstantially parallel adjacent grid wires thereby maintaining aconstant filament-to-grid spacing.

BACKGROUND OF THE INVENTION This invention relates to electron tubefilament structures and particularly to a novel filament structure whichprovides improved stability of interelectrode spacing during tubeoperation.

In electron tubes having a directly heated cathode, the electrodestypically comprise a filamentary cathode, one or more grids, and ananode. The filamentary cathode conventionally consists of a number ofparallel, thoriated tungsten filaments arranged in horizontally spaced,vertically extending orientation to form a cylindrical or caged pattern.The grid comprises a number of non-emissive wires similarly arrangedinto the shape of a cylinder and disposed coaxially with the filamentarycathode. A copper anode in the shape of a hollow cylinder is locatedcoaxially about the grid. Though each electrode is radially spaced fromeach other, the cathode-to-grid spacing is quite small, frequentlymeasuring only some ten to some two hundred thousandths of an inch.

Typical filamentary cathode and grip supports provide upper and lowermembers to which the ends of the filaments and grid wires arerespectively fastened. Though both members of the grid support arefixed, only one member of the cathode support, usually the lower, isfixed while the other is biased therefrom by a spring. In this mannerthe filaments are maintained in a state of tension throughout the rangeof temperatures encountered during storage and tube operation.

Were the tension maintained on each filament the same as upon eachother, the bias alone would be sufficient compensation for theelongations and contractions of the filaments caused by their extremechanges in temperature. This, however, is not actually the case. Inpractice it is impractical if not impossible to mount all the filamentsin an identical manner. Furthermore, the physical geometry of eachfrequently varies ever so slightly. There are also latent strains andstresses in the filament supports which may be relieved at elevatedtemperatures causing nonuniform and unpredictable elongations andcontractions. These variations, though tenuous, are frequently of enoughsignificance as to cause one or more filaments to have insufficienttension at operative temperatures. In this situation, further expansionof such a filament will not be compensated longitudinally by means ofthe support bias. Expansion will thus cause the filament to movelaterally, that is to bow. This bowing can take place in any direction,the vast majority of which "Ice have a component normal to the filament.In so bowing the interelectrode spacing between the filament and theadjacent grid wire is altered. As this spacing is critical to propertube operation, any change adversely affects the performance of theelectron tube. In the more extreme cases of bowing, the filament willactually touch an adjacent grid wire causing an electrical short circuitand probable tube destruction.

Heretofore, the above described problem has been partially abatedthrough the use of filament supports having separate resilient membersto which each individual filament is fastened. With this structure,divergent forces arising in each filament from thermal expansion areindependently compensated. The prior art approaches to this problem hasalmost universally been directed towards various designs in thesefilamentary supports. The theme has been to impart general tensioning ofthe filaments as a whole through the use of a spring, and then tocompensate for variations in individual filaments through the use ofsecondary biasing means in the filament support. Some supports, known inthe art as spiders, have taken the form of a thin disk having radiatingarms to which individual arms are held. These arms, which typically havesignificant widths, are made of molybdenum. The arms thus have moreresiliency than the filaments which are made of thoriated tungsten. Insome designs, a plurality of arms having varying degrees of resiliencyhave been used; in others, split arms have supported a filament simplylooped over both sections of the arm. But in all of these designs ofvarying complexity, the central thought has prevailed: Impart resiliencyinto the filament support to provide secondary biasing means to damperadverse variations in the tensioning of individual filaments.

Accordingly, it is an object of the present invention to provide anelectron tube filament structure which produces improved stability ofinterelectrode spacing under varying thermal conditions.

Another object of the invention is to provide a novel electron tubefilament configuration which self-compensates for deviations from apredetermined norm of tension.

An associated object of the invention is to provide a simplified, rigidfilament support which may be uniformly and inexpensively fabricated andmounted.

SUMMARY OF THE INVENTION Briefly described the present invention is anelectron tube filament structure comprising a filament wire and afilament support. The filament wire has two rectilinear strands joinedat one end by a unitary bight. Attaching means affix one end of eachstrand in the tube while the filament support abuts the filament bight.The filament support is biased from the attaching means to tension thetwo rectilinear strands to a predetermined degree. The portion of thebight which abuts the filament support does not exceed approximatelyhalf of the total length of the bight whereby deviations in tension of arectilinear strand are dampened through change in the configuration ofthe bight.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is an elevational view of anelectron tube, portions of which are broken away revealing oneembodirnent of the filament structure of the present invention.

FIG. 2 is a perspective view of one embodiment of a filament supportmade in accordance with the present invention.

FIG. 3a is an elevational View of a typical filament and filamentsupport combination of the prior art. FIGS. 3b, 3c, and 3d areelevational views of three embodiments of the present improvement overthe prior art structures.

FIG. 4a is an extended elevational view of the embodiment shown in FIG.30 with an associated force diagram. FIG. 4b diagrammaticallyillustrates the result of adverse, filamentary tensioning. FIG. 40 is apartial plan view of filamentary and grid wires in association with adisplacement diagram.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now in more detail toFIG. 1 there is illustrated an electron tube of the 3CW20,000 generalseries, and in particular the EIMAC Triode 3CW20,000H3. The cut-awayportion of the tube reveals in cross section an outer envelope housing acup-shaped copper anode 12 having a tubular pinch-off 14 extendingthrough the anode and envelope about the tube axis. The pinch-off isenclosed by a tubulation cap 16 which is afiixed to envelope 10. Housedwithin anode 12 are a molybdenum grid cap 18 which supports molybdenumgrid wires 20. Though only two grid wires have been illustrated for thesake of clarity, it should be understood that grid Wires 20 are held allabout the circumference of grid cap 18 to form a cylindrical cage. Thelower ends of the grid wires are afiixed to a grid support 21.

The cathode consists of a plurality of thoriated tungsten filamentswhich are centrally bent to form bights, each of which integrally jointwo rectilinear strands 26. One free end of each rectilinear filamentstrand 26 is welded to a finger 30 which extends from an inner filamentsupport 32. The other free end of each rectilinear strand is welded toan adjacent finger 31 extending from an outer filament support 22. Boththe inner and outer supports and their fingers are made of nickel. Eachfilament bight 24 is looped over a cantilevered arm 34 of a centrallyapertured spider. A hub with radially extending rods 34' assembly, asillustrated in FIG. 2, may be substituted for the spider and armsfilament support. Such a hub and rods support is known as theWagonwheel.

Near the free end of each arm 34 or rod 34' is a notch 38 into which thebight lays and is kept from sliding axially along the arm or rod. Once afilament has been looped over the notch and the ends welded to fingers30 and 31, rectilinear strands 26 are aligned essentially parallel toeach other.

Also shown in the cut-away portion of the tube are means for biasing theupper filament support away from lower filament supports 22 and 32. Oneend of a center molybdenum rod 40 is held in a recess 42 within copperhousing 44. This housing also retains a spring 46 coaxially about centerrod 40. Spring cups 48 are fitted about each end of the spring. Spacers50 are inserted between the lower spring cup from the inner floor of thehousing. A hollow, pusher rod 52 is placed coaxially about center rod 40and affixed at one end to the upper spring cup. A hollow, cylindricalgetter 41 is then placed about the pusher rod. Finally a dielectriccollar 54 mechanically links but electrically insulates the upper end ofpusher rod 52 from the spider and arms 34. Thus spring 46 exerts anupward force on the spider which tensions filament strands 26 aligningthem in parallel. The degree of tension may be adjusted through theinclusion or exclusion of spacers 50. Approximately 3 pounds of tensionis all that is typically required. The upper filament support iscoaxially aligned with the grid by center rod 40 which extends through acentral aperture in the spider.

The ease with which the described filamentary cathode and its supportmay be assembled should be appreciated. Once the free ends ofrectilinear strands 26 have been welded to fingers 30 and 31, spring 46may be manually depressed and filaments the bights easily slipped overthe ends of arms 34 and into notches 38. Spring 46 may then be manuallyreleased thereby positioning all the filaments.

Reference to FIG. 2 will disclose the simplicity of the alternate upperfilament support. This assemblage merely consists of an apertured hub towhich a plurality of rods are radially attached. The rods need not beindividually adjusted or tensioned, nor need they have resiliency.

Turning now to FIG. 3a there is illustrated in elevation the upperportion of a filamentary wire and that portion of its upper support bywhich it is directly supported. The filament is looped over and pressedagainst a mandrel having a rectangular cross-section causing it to bebent into two rectilinear strands 56 and a rectilinear bight 58 withright angled junctures 60 therebetween. The unitary filament is thenplaced over a filament support finger 62 having a cross-sectionessentially congruent with that of the mandrel. This filament structureis that in the prior art most closely resembling that of the presentinvention, It is here illustrated for comparative purposes in order thatthe significance of the present invention can be easily and fullyappreciated. It is recognized that at first glance the structure of thepresent invention does not appear grossly distinctive over the prior artof FIG. 3a. A careful examination of the embodiments of the presentinvention shown in FIGS. 3b, 3c, and 3d will reveal, when read incontext with the explanations presented in FIG. 4, the very significantphysics resulting from the slight alterations in filament configurationwhich in turn result in greatly improved stability of inter-electrodespacing.

FIG. 31; illustrates a unitary filament having two rectilinear strands64 and an interconnecting bight 66 looped over a filament support finger62 of the same crosssection of that illustrated in FIG. 3a. In thisembodiment bight 66 has a rectilinear midsection which abuts the uppersurface of a notch in finger 62. At each end of the midsection the bightarcs for approximately at which point it integrates into parallel,rectilinear strands 64. Note that the breadth of finger 62 is slightlyless than half the span between strands 64. Also note that no section ofthe bight nor of the rectilinear strands abuts the sides of the supportfinger.

Another embodiment of the present invention is shown in FIG. 30. Herethe filament has two rectilinear strands 64 integrally connected by abight. The center portion of the bight has an are 68 which is loopedover a notch in rod-shaped finger 70. Each end of are 68 merges into ashort rectilinear portion of the bight, the other ends of whichintegrally merge into other arcs 69 which in turn integrate with theends of rectilinear strands 64.

Such a configuration conforms to the inner contour of an arch termed byarchitects a tudor arch. Alternatively are 68 may be crimped in whichcase the bight generally conforms to the inner contour of a structureknown as an ogee arch. Note in both alternatives that only a smallportion of the overall bight actually abuts support finger 70.

FIG. 3d illustrates yet another embodiment having two rectilinearfilament strands 64 interconnected by a bight 71 looped over a notch inrod-shaped support finger 70. In this example, bight 71 assumes the formof a semicircle or round arch.

FIG. 4 illustrates the physics. experienced by the configurations ofFIGS. 3b, 3c and 3d, due to thermal expansion during tube operation. InFIG. 4a there is again illustrated the filament structure of FIG. 30together with fingers 30 and 31 to which the free ends of a filament arewelded. The force of spring 46 transmitted to the rodshaped finger 70 isshown by vector S. This force is distributed to two reactant forces inthe two rectilinear strands 64 illustrated by the force vectors T whichbring the filament support into equilibrium. As force S is distributedamong some 40 filaments, force T in any single filament is quite small,just sufiicient to align filaments 64 in parallel. It should beappreciated that force S must not become excessive so as to alter theconfiguration of the bight itself at nonoperative temperatures.

Now suppose for a moment that upon heating of the filament structureduring tube operation an imbalance in forces T is created. This wouldresult from the length of one rectilinear strand being different fromthat of the other strands once all have thermally expanded. This couldbe caused by several factors including the thermal release of latentstresses or strains in finger 30 or 31, nonconformative in thefilamentary wire used, as in the slight slippage of a free end of onestrand when it is welded to a finger. A very small deviation in thelength of the filament and its associated tension can have verysignificant alteration in the interelectrode spacing between thefilament and adjacent grid wires. For example, should the length of eachstrand 56 in the prior art filament structure of FIG. 3a be specified as10.0 cms., and instead should one strand become 10.1 cms. through anerror in mounting, then the erroneously mounted strand will bowlaterally a distance of approximately 1 /2 cm. at its midpoint. This canbe more easily visualized in FIG. 4b where A is the predetermined length10.0 cm. and A is the actual length 10.1 cm. By the pythagorean theoremB, which represents the maximum degree of bowing, becomes 1.4 cms. Thusa longitudinal error of just 1 part in 100 produces a lateral error ofapproximately 14 parts which could easily be sufficient to completelytraverse the spacing between the filament and an adjacent grid wirethereby causing an electrical short. But of course any significantdeviation in this spacing is detrimental to the operation of theelectron tube since the cathode-to-grid spacing is both quite close andcritical. Variation from the designed spacing is one of the chief causesof power grid electron tubes failing to meet operative specifications.

In the case where a filament strand after expansion is shorter than theother strands, the filament of FIG. 3a does not provide any alternativeconfiguration which the filament could assume. Rather support arm 62 hasbeen designed with a degree of resiliency. Thus where one filamentstrand is under more tension than the others, arm 62 has bentdownwardly. In doing so, however, the end of the arm to one which thefilament is hung pivots slightly away from the adjacent grid wires. Thisof course alters the interelectrode spacing between the cathode and gridresulting in a change in tube perveance. Should the resiliency of arm 62not ofiset the abnormal tension in the particular filament strand, theentire spider to which the arm integrates will be held too close to theother filament support. This is to say that but one arm 62 will becounterbalancing the biasing force of spring 46. In this situation thefilaments held to other arms 62 will not be taut; they will then be atliberty to bow towards or away from the adjacent grid.

Though we have discussed the lateral displacement that may occur when afilament strand bows, we have yet to discuss the direction in which thebowing takes place. In FIG. 40 there is illustrated a plane view, a setof filaments 72 and a set of grid wires 74. About one of the filamentsis drawn a dotted circle 72'. This circle, for the sake of illustration,is presumed to define the lateral bounds a bowed filament assumes atelevated temperatures. Should this bowing occur along the are 76 definedby the cylindrical cathode, the slight decrease in spacing between thebowed filament and one of the adjacent grid wires 74 is approximatelyoffset by the decrease in its spacing with the other grid wire 74. Thisdoes not cause a significant alteration in the perveance of the tube.However, should such bowing occur in any direction other than along are76, the spacing between filament 72' and grid wire 74 would change. Theconfiguration illustrated in FIGS. 3b, 3c, and 3d serve to prevent thisoccurrence. In these figures each pair of rectilinear strands 64 definea plane which forms a segment of are 76. Should one of the strands 64 inFIG. 4a, for example, be mounted too low on fingers 30 and 31, the bightwill take the shape illustrated by the dotted line x in FIG. 4a; shouldit be mounted too high on the finger it will take the shape of dottedline y. In this later case support rod 70 will move very slightly upwardbut this has not been shown for the sake of clarity.

These described deviations from the designed norm occur in the planedefined by the two strands 64, i.e., in the plane of this paper in FIG.3. In this manner the excess tension is relieved through a change inbight configuration in a .predesignated plane. Likewise any slightbowing is confined to this same plane. This controlled direction inbight deviation occurs along are 76. In this manner the electrodespacing is maintained.

It should be understood that the above-described embodiments are merelyillustrative of applications of the principles of the invention.Obviously many modifications may be made in the specific example withoutdeparting from the spirit and scope of the invention as set forth in thefollowing claims.

What is claimed is:

1. An electron tube having a filament structure comprising a filamentwire and a filament support, said filament wire having a bight and tworectilinear strands, means attaching an end of each strand in the tube,said attaching means aifording electrical connection to the respectiveends, said filament support being relatively rigid and mounted in spacedrelation to said attaching means and supporting said filament wire byabutting a portion of said bight, said filament support being biasedaway from said attaching means to tension the rectilinear strands to apredetermined degree, the abutting portion of said bight not exceedingapproximately half the total length of the bight, and the remaininglength portion of said bight being shaped and dimensioned relative tosaid support to form a deformable arch portion, whereby deviations intension of a rectilinear strand from said predetermined degree aredampened through deformation of said deformable arch portion of saidbight.

2. The election tube of claim 1 wherein said filament support is a rigidcantilever.

3. The electron tube of claim 1 wherein said filament support comprisesa metal selected from the group consisting of molybdenum and tungsten,and wherein said filament wire comprises thoriated tungsten.

4. The electron tube of claim 1 wherein said filament support iscylindrical.

5. The electron tube of claim 4 wherein said rectilinear strands aresubstantially parallel and wherein the diameter of saidcylindricalfilament support does not exceed half the spacing between thetwo strands.

6. The electron tube of claim 1 wherein said filament support is aparallelepipedon and wherein said bight abuts but one surface of saidparallelepipedon.

7. The electron tube of claim 1 wherein said filament support is aparallelepipedon and wherein said bight abuts the paralllelepipedon onlyat two edges thereof.

.8. The electron tube structure of claim 1 wherein the filament supportcomprises a hub disposed perpendicular to the force of said bias and aplurality of spokes radially afiixed to said hub.

9. The electron tube of claim 8 having a plurality of said filamentwires wherein said spokes are cantilevers each of which support afilament wire bight.

10. The electron tube of claim 9 wherein said spokes have a notch neartheir free end into which said filament wire bights are placed and heldto the spoke.

11. The electron tube of claim '8 having a spring, a pusher having oneend fixed to said spring and a dielectric spacer fixed to the other endof said pusher abutting said hub.

12. The electron tube of claim 11 wherein said hub and said dielectricspacer have aligned apertures and wherein said filament structure is incombination with a second filament support having an element coaxialwith said pusher extending through the apertures in said hub anddielectric spacer.

13. The electron tube of claim 1 wherein the configuration of said bightis the inner contour of a round arch.

14. The combination of claim 1 wherein the configuration of said bightis the inner contour of a tudor arch.

7 8 a 15. The combination of claim 1 wherein the configura- 3,218,50211/ 1965 Freggens 313-278 X tion of said bight in the inner contour of around arch. 3,299,310 1/ 1967 Freggens 313-278 16. The combination ofclaim 1 wherein the coniignra- FOREIGN PATENTS t1on of sald bightconsists of two arcuate sections oined together by a rectilinearsection. 1331911 1/1946 Austraha- 5 73,735 2/ 1952 Denmark. uNlTg s zgfrl is gjfTENTs JOHN W. HUCKERT, Primary Examiner. 2 385 9/194 1 A. J.JAMES, Assistant Examiner ,435 5 W t 313-278 a a 10 US. 01. X.R.

2,602,907 7/1952 Shower 313-278 2,632,129 3/1953 Dailey 313278 X 313271,275, 277

